Earthquakes and Tsunamis

The governing concept is the tectonic hazard chain at a subduction zone. At an ocean trench, one plate may descend beneath another, locking parts of the boundary and allowing elastic strain or stress to accumulate. When the fault suddenly slips, an earthquake occurs. If the seafloor moves vertically, it displaces the overlying seawater; the disturbance can travel outward as a tsunami. Thus option A gives the correct sequence, although not every trench earthquake produces a tsunami. The other choices describe the water cycle, fluvial deposition and aeolian coastal processes, none of which explains trench-related seismic risk.

The governing concept is tsunami generation by rapid displacement of the ocean-water column. A trench marks a deep convergent-margin setting, but trench presence alone does not create a tsunami. A large, shallow submarine earthquake, especially one that produces vertical uplift or subsidence of the seabed, suddenly moves the overlying water and sends long waves outward. Therefore, option A is correct. A change in sea colour is not a physical trigger, an abyssal plain is simply a broad deep-sea surface, and blowing coastal sand is an atmospheric or coastal process unrelated to the initial tsunami mechanism. The most decisive evidence is sudden seafloor displacement.

A tsunami is a series of long waves produced by the sudden displacement of a large volume of seawater. The displacement may result from an undersea earthquake that shifts the sea floor, although submarine landslides, volcanic activity, and rare impacts can also generate tsunamis. Normal wind-generated waves are created mainly by energy transferred from moving air to the ocean surface and generally affect a shallower surface layer. In deep water, a tsunami may have a long wavelength and low visible height, but near the coast it can slow, grow higher, and inundate land. Option A gives the correct distinction; B, C, and D confuse unrelated processes.

A submarine earthquake can abruptly displace the seafloor and the overlying water column. This disturbance may generate a tsunami, a sequence of waves with very long wavelengths that can travel rapidly across deep water. As the waves enter shallow coastal water, they may slow, compress, and grow much higher, creating severe coastal flooding and currents. Thus option C is the correct identification, although confirmation requires monitoring and official warnings. Spring and neap tides are predictable gravitational conditions associated with the relative positions of the Sun, Moon, and Earth, not earthquake shocks. A normal wind wave is generated by surface wind and does not best explain this clue.